1. Field of the Invention (Technical Field)
This invention relates to improved apparatuses and methods for remelting metal alloys.
2. Background Art
Vacuum arc remelting ("VAR") is a melting and solidification process used to produce high quality ingots of chemically reactive or segregation sensitive alloys. The alloy is cast or forged into an electrode, and then remelted and solidified in a vacuum. A sustained high (several kiloamperes) direct current is used to induce an electrical arc between the electrode and a conductive container. Energy from the electrical arc melts the electrode (which, as mentioned, is cast from the alloy to be remelted) into the container.
It is known in the art that VAR improves the quality of alloys subjected to its processes. Among other things, the following improvements in the produced ingot have been noted:
(1) Contained gases, especially hydrogen and oxygen, are reduced; PA1 (2) The alloy is cleaner (fewer non-metallic inclusions); PA1 (3) Center porosity and segregation in the ingot are greatly reduced; and PA1 (4) Mechanical properties of the remelted alloy, such as ductility and fatigue strength, are improved.
Presently, VAR is the most commonly used melting process used to produce ingots for many wrought alloy applications. VAR is particularly well-suited to melting nickel-based "superalloys" (such as Alloy 718) which contain substantial quantities of reactive elements, because melting is performed in a vacuum and the solidification environment can be controlled to the optimum. It is also known that equilibrium phase relationships dictate the solutal partition at the solidification interfaces, and that local conditions about the arc zone thus determine the chemical homogeneity of the produced ingot. More particularly, the control of pressure and/or the composition of the gas over a melt makes it possible to deoxidize the melt with carbon or hydrogen, which in turn produce gaseous deoxidation products, which, if removed, can reduce the formation of solid non-metallic inclusions in the produced ingot.
The design and application of VAR have evolved to appreciable levels, as described in U.S. Pat. No. 4,450,570 to Weingartner et al., and patents referenced therein. Nevertheless, the lack of a thorough understanding of the metal vapor arc and its relationship with the metallurgy of the VAR process have hampered the production of ingots with rigorous quality standards. Few improvements have been made in the common VAR furnace to substantially increase its capacity to purify alloys through remelting. The apparatuses and methods of the present invention, through the application of heightened understanding of the conditions at the solidification interfaces, aid substantially in the production of higher quality ingots than those produced in furnaces common in the art.